Gas turbines have an operating temperature increased year by year for the purpose of improving the efficiency. In order to deal with the increase in operating temperature, a thermal barrier coating (hereinafter referred to as a “TBC”) made of ceramic is applied to the surface of a gas turbine hot part to relieve the environmental temperature of the part. In general, the application of the TBC can decrease the temperature of a substrate by 50 to 100° C. depending on the conditions of use.
Japanese Unexamined Patent Application Publication No. SHO62-211387 discloses a TBC including a thermal barrier layer made of partially-stabilized zirconia with low heat conductivity and excellent heat resistance. The TBC is formed over the substrate via a MCrAlY alloy layer, where M represents at least one element selected from the group consisting of iron(Fe), Ni, and Co, Cr represents chrome, Al represents aluminum, and Y represents yttrium.
The TBC for the gas turbine needs to improve the heat resistance so as to deal with the high-temperature combustion gas, and to reduce granular friction (erosion) of solid particles (mainly, oxides) contained in the combustion gas. Japanese Unexamined Patent Application Publication No. HEI9-78258 discloses that a dense protective layer is provided on the surface of a porous thermal-barrier layer with excellent heat resistance for the purpose of improving the resistance to erosion.
The gas turbine has conventionally used relatively high-grade oil as fuel, such as liquid natural gas (LNG), kerosene, or light oil, from the viewpoint of combustion characteristics and corrosion prevention. Such fuels contain very small amount of corrosion factor, such as sulfur(S) and ash content. The approximate amount of S is equal to or less than about 0.01% by mass and the amount of ash content is nearly equal to zero. Therefore, the hot part hardly undergoes corrosion and damage.
In recent years, however, from the viewpoint of increasing cost of fuel and saving resources, the gas turbines have increasingly used the low-grade oil, such as heavy oil. The low-grade oil contains elements as corrosion factors, such as sulfur, alkali metal, or vanadium. In combustion gas, the corrosion factor elements react with each other, with oxygen (O) in combustion air, or with sea salt particles (for example, NaCl) in a complicated manner to generate a compound which causes corrosion at high temperature. Such a compound causes damage to the TBC, especially a zirconia layer part of the TBC.
The object of the present invention is to provide a high-temperature resistant component for a gas turbine hot part and so on. The high-temperature resistant component has durability and reliability enough to withstand a corrosive environment with use of the low-grade oil.